NORFOLK, Virginia (AP) — Federal officials identified a virus Tuesday as the likely reason hundreds of bottlenose dolphins died along the East Coast, but they say there’s little they can do to stop the deaths.

More than 330 dolphins have been stranded between New York and North Carolina since July 1, with nearly all of them dead by the time they wash up on shore, the National Oceanic and Atmospheric Administration said.

That’s more than nine times the historical average for dolphin strandings in the region during July and August.

“Along the Atlantic seaboard, this is extraordinary,” Teri Rowles, NOAA Fisheries Marine Mammal Health and Stranding Response Program coordinator, said in a conference call with reporters.

Earlier this month, NOAA declared an unusual mortality event so it could provide additional resources to study what was behind the rapid increase in deaths — more than half of which have occurred in Virginia. At the time, they suspected the cetacean morbillivirus was causing the deaths, just as it did during the last major dolphin die-off. In 1987 and 1988, the virus was blamed for causing 740 dolphin deaths between New Jersey and Florida.

Although research will continue, NOAA said it has collected enough evidence to declare the virus as the “tentative cause” in the most recent string of deaths as well. Morbillivirus is found in a broad range of mammals, and dolphins with it typically experience symptoms such as skin lesions, brain infections and pneumonia. The virus is usually spread through inhalation of respiratory particles or direct contact between animals, although officials said there’s no risk of humans catching it. Bottlenose dolphins are typically found in groups of two to 15.

Officials at the Virginia Aquarium & Marine Science Center said many of the dolphins washing up on the state’s beaches are badly decomposed. State and federal officials say there are untold numbers of other dolphins that have also died and haven’t washed ashore, likely making the total death count much higher.

Using the 1980s die-off as a guide, officials believe the disease and strandings will spread south and last through the spring of 2014. Eventually, remaining dolphins will become more resistant to the disease, just as they have before. Bottlenose dolphins typically live between 40 and 50 years, but a new generation of dolphins will also likely become susceptible to the disease again in the future.

There are two different stocks of dolphins that populate the affected region, with the northern stock having between 7,000 and 9,000 dolphins, while the southern stock has between 9,900 to 12,000 dolphins, according to federal estimates.